In conventional optical scanning devices, polygon mirrors are widely used as optical deflectors for scanning light beams. A polygon mirror is rotated at high speed to scan a light beam. To form an image of increased resolution and at increased speed with the use of a polygon mirror, the polygon mirror needs to be rotated at increased speed. However, in order to rotate the polygon mirror at high speed, it is necessary to increase the durability of the bearing and mitigate problems of heat and noise. Thus, there is a limit to high-speed scanning with the use of a rotating body including a mirror.
Meanwhile, in recent years, there has been proposed an optical scanning device for scanning light beams having a configuration in which a micromirror is oscillated by a silicon micro-machining technique. Such micromirror devices can be classified in terms of the driving method. For example, micromirror devices employing an electromagnetic driving method and an electrostatic driving method have been proposed.
For example, patent document 1 (Japanese Laid-Open Patent Application No. 2002-78368) proposes a method using a magnetic field generating unit. Patent document 2 (Japanese Laid-Open Patent Application No. H8-211320) proposes a method using an electrostatic induction generating unit.
In the conventional proposals, when the driving method using the magnetic field generating unit or the driving method using the electrostatic induction generating unit is employed as the method of driving the micromirror movable unit, the driving voltage is steadily applied as sine wave alternating current signals to drive the micromirror movable unit.
A typical example using the electrostatic induction generating unit is disclosed in patent document 3 (Patent No. 3011144), in which the optical scanning device oscillates a mirror with electrostatic attraction.
The structure and operation of a conventional oscillating-type mirror are described with reference to FIG. 14.
FIG. 14 shows a plan view and a cross-sectional view of an oscillating mirror. As shown in FIG. 14, a mirror 102 is disposed in a recessed part of a supporting substrate 101, and the mirror 102 is supported by the supporting substrate 101 via a torsion bar 103 which is integrated with the mirror 102. According to a torsional function of the torsion bar 103, both sides of the mirror 102 can be oscillated in a perpendicular direction with respect to the plane of the mirror. The torsion bar 103 is made of a conductive member, and both edges of the torsion bar 103 are electrically connected to pads 104 provided in the supporting substrate 101. Furthermore, fixed electrodes 107 are supported via insulators 106 on both sides of the recessed part of the supporting substrate 101. The fixed electrodes 107 are disposed in such a manner as to be at higher positions along the oscillating direction with respect to initial positions of mirror electrodes provided on both sides of the mirror 102. At the initial positions of mirror electrodes, the mirror electrodes and the fixed electrodes are disposed in such a manner as to have a difference in height with respect to each other.
In the optical scanning device using this oscillating-type mirror, a high voltage is applied between pads 108 of the fixed electrodes 107 and the pads 104 to which the torsion bar 103 is connected. Accordingly, an electrostatic force is generated between the fixed electrodes 107 and the mirror 102. Due to this electrostatic attraction, one of the sides of the mirror 102 is attracted toward the fixed electrodes 107. This attracting action causes the torsion bar 103 to twist and deform so that the mirror 102 oscillates in a perpendicular direction with respect to the plane of the mirror. Immediately after this oscillating action, the applied voltage to the fixed electrodes 107 is stopped, so that the mirror oscillates in the opposite direction due to the twist restoring force. By repeatedly applying and not applying the voltage, the mirror 102 can be oscillated, and the oscillated mirror 102 reflects a light beam from a light source (not shown) to deflect and scan the light beam.
Incidentally, two kinds of electrostatic driving methods are presently used. In one of the methods, the driving electrodes have a parallel plate electrode configuration. In the other method, the driving electrodes have a comb teeth-shaped electrode configuration. The comb teeth-shaped electrode method is generally said to be significantly superior to the parallel plate electrode method in terms of the amount of movement and the driving force. The method using comb teeth-shaped electrodes is disclosed in, for example, patent document 4 (Japanese Patent No. 3006178), patent document 5 (Japanese Laid-Open Patent Application No. H5-224751), and patent document 6 (Japanese Laid-Open Patent Application No. 2003-241120).
When an optical deflector for scanning light beams is used to form an image by scanning a light beam, the variation in the scanning speed, i.e., the variation (jitters) of the scanning time for scanning an arbitrary distance, generally needs to be less than or equal to 0.02%. The aforementioned micromirror is known to vary in terms of oscillation amplitude in accordance with the variation of the environment in which it is being used. The variation in the oscillation amplitude of the micromirror causes variations in jitters of the light beam scanning operation, which leads to degraded image quality. Many methods have been proposed in an attempt to solve this problem. In a typical method, the jitters are measured, and based on the variation amount of the jitters, the variation of the oscillation amplitude of the micromirror is estimated. Then, the energy applied to the micromirror is adjusted in such a manner as to correct this variation.
Patent document 7 (Japanese Laid-Open Patent Application No. 2005-208460) discloses a method for overcoming the problem of degraded image quality caused by the variation in the resonance frequency of a deflection mirror. Specifically, this method is for adjusting the driving frequency or the oscillation amplitude of the deflecting mirror. That is, the deflecting mirror is directly controlled to maintain image quality.
Furthermore, in patent document 8 (Japanese Patent No. 3584595) and patent document 9 (Japanese Patent No. 3543473), methods of mitigating the image quality from being degraded, which is caused by variations in the deflection frequency, are realized by providing a frequency measuring unit and a time adjusting unit to adjust the light beam irradiation time. That is, with these proposed methods, even if the oscillation frequency of the deflecting mirror varies, the image quality can be improved without directly adjusting the unit for driving the deflecting mirror.
The above cases describe that degraded image quality is caused by the variation in the oscillating frequency of the deflecting mirror. However, characteristics of the optical scanning device using the deflecting mirror developed by inventors of the present invention were evaluated, and it was found that the variation in the oscillating frequency, i.e., the frequency jitter, was less than or equal to 0.003%, and the amplitude variation was larger than this frequency jitter. The measured value showed that the jitter of the light beam caused by amplitude variations was 0.2%, which leads to pixel displacements corresponding to four displaced pixels at a pixel density of 1,200 dpi (dots per inch).
Thus, it was determined that degraded image quality is most likely caused by the oscillation variations in the optical scanning device using the deflecting mirror being developed by the inventors of the present invention.
Patent document 1: Japanese Laid-Open Patent Application No. 2002-78368
Patent document 2: Japanese Laid-Open Patent Application No. H8-211320
Patent document 3: Patent No. 3011144
Patent document 4: Japanese Patent No. 3006178
Patent document 5: Japanese Laid-Open Patent Application No. H5-224751
Patent document 6: Japanese Laid-Open Patent Application No. 2003-241120
Patent document 7: Japanese Laid-Open Patent Application No. 2005-208460
Patent document 8: Japanese Patent No. 3584595
Patent document 9: Japanese Patent No. 3543473
Conventionally, in an attempt to overcome the above-described problems of the background art, the above-described units for directly adjusting the oscillation amplitude of the deflecting mirror are used for mitigating variations in the deflecting mirror. However, the deflecting mirror being developed by the inventors of the present invention has a large time constant as the response characteristic. Thus, it is difficult to adjust such a high-speed deflecting mirror, and there is a limit to preventing degraded image quality by adjusting the oscillation amplitude.
Accordingly, there is a need for an optical scanning device using a deflecting mirror capable of preventing the image quality from being degraded even when oscillating variations occur, by appropriately adjusting a light beam to turn ON/OFF.
Specifically, there is a need for an optical scanning device capable of mitigating degradation of image quality even when oscillating variations occur in the deflecting mirror, by controlling image formation on an image carrier in accordance with the oscillating variations, and there is also a need for an image forming apparatus including such an optical scanning device capable of forming favorable images.